The present invention relates to diversity receivers.
Conventional television signals transmit video information in analog form by modulating the amplitude and frequency of a carrier signal. Digital television systems convert an analog video signal into digital information, which is transmitted by pulse modulating the amplitude and phase of a carrier signal. For example, in broadcasting high definition television (HDTV), digital information is transmitted by an 8 level amplitude modulation technique, known as 8 VSB (vestigial sideband). In 8 VSB, modulation of a carrier to one of 8 levels (i.e., one of 8 symbols) defines 3 bits of digital information for each symbol clock interval. While analog television signals degrade gracefully in the presence of interference, digital broadcast systems can fail completely when the bit error rate overcomes the error tolerance of the system. Bit errors result from weak signals, noisy signals or signals subject to fading and distortion.
In an ideal radio wave propagation environment, there exists an unobstructed line-of-sight path between the transmitting and receiving antennas. Additionally, no other objects exist which may reflect the transmitted wave along another path to the receiving antenna. As is more often the case, however, there is no direct line of sight between the antennas. In an outdoor environment, natural or artificial obstructions, such as buildings, hills, and trees block the direct line of sight. Furthermore, these obstructions reflect the transmitted signal such that multiple versions with varying amplitudes, phases and time delays are simultaneously received.
The indoor environment is even more complicated since there is rarely an unobstructed path between the transmitter and receiver antennas. Furthermore, objects causing signal reflection and absorbtion are numerous and are often in motion.
Reception of the transmitted signal along multiple paths from the transmitter to the receiver causes signal distortion which manifests itself in a variety of ways. The different paths have different delays that cause replicas of the same signal to arrive at different times (like an echo) and sum at the receiver antenna, causing inter-symbol interference. The phases of these multipath signals may combine constructively or destructively resulting in large range of possible signal strengths. Additionally, this signal strength may vary with time or antenna location, and is known as signal fading. Signal fading can range from frequency selective fades to a flat fade over the entire frequency spectrum of interest. Indoor signals typically have severe multi-path distortions and are changing rapidly in time, ranging from flat fades to deep in-band nulls to relatively unimpaired signals. Conventional indoor TV antennas and outdoor antennas used with existing 8 VSB receivers often do not produce reliable and uninterrupted reception of digital broadcast HDTV signals.
To mitigate the adverse effects of multiple path (multipath) distortion and signal fading, it is known to use a diversity receiver. In a diversity receiver, two (or more) independent antennas are used to receive two (or more) separate versions of the same signal. Each independent antenna provides a signal with different (ideally, uncorrelated) noise, fading and multi-path factors. These different signals may be obtained through various forms of diversity including spatial, temporal, polarization and direction-of-arrival diversity.
A diversity receiver has a plurality of receiver channels to process the plurality of antenna signals. By appropriate combining of the information extracted from each signal by each channel of the diversity receiver, a diversity receiver provides equal or superior performance compared to a non-diversity receiver operating on the xe2x80x9cbestxe2x80x9d of the received signals alone.
In the prior art, the received signals in each of the individual receiver channels of a diversity receiver are processed separately and then combined. That is, each of the multiple receiver channels in a prior art diversity receiver is an independent receiver. Each of the respective diversity antenna signals is processed in one of the independent receiver channels of the diversity receiver.
Each receiver channel is independent in the sense that each includes a respective separate tuner, front-end function (such as for baud clock recovery and carrier recovery) and separate equalizer filter. The separate receiver channels of the prior art provide separate signal outputs, which are then combined in some manner.
In one prior art approach, the output of the separate diversity receiver channel having the stronger input signal (i.e., the higher signal to noise ratio) is selected over the output of the diversity receiver channel having the weaker input signal. In a second prior art approach, the outputs of the two diversity receiver channels are combined equally, regardless of input signal strength. In a third prior art approach, the outputs of the two receiver channels are combined in a maximal ratio combiner in accordance with the respective signal to noise ratio of each signal. In a maximal ratio combiner, the receiver channel with the highest signal to noise ratio provides the greatest contribution to the final output. In general, a prior art diversity receiver processes the received diversity signals in separate receiver channels with regard to receiver functions such as tuning, automatic gain control (AGC), baud clock recovery, RF carrier recovery, and forward equalization.
In one embodiment of a diversity antenna system for use in conjunction with the present invention, first and second identical antennas are separated, but oriented identically within a plane. The first and second antennas are separated by several wavelengths to provide respective first and second RF signals; The first and second RF signals are said to be reception by the use of spatial diversity since it is known that the multipath propagation channels (in a parameterized sense) from the transmitter to the first antenna and from the transmitter to the second antenna are nearly uncorrelated.
The present invention is embodied in a multiple channel diversity receiver with joint signal processing. In particular, the first and second RF signals are processed jointly in a multiple channel diversity receiver with respect to tuning, automatic gain control (AGC), baud clock recovery, RF carrier recovery and forward equalization. Joint processing, as compared to independent processing of the prior art, means that the multiple channels of the diversity receiver are linked or cross coupled to each other through various joint processing circuitry.
TUNING
In accordance with the present invention, the first and second RF signals are processed jointly in the multiple channel diversity receiver with respect to tuning. In particular, the first and second tuners in the first and second channels of the multiple channel diversity receiver share at least one joint local oscillator. In the case of a dual conversion tuner, first and second joint local oscillators are shared. A first joint local oscillator is shared in the RF stage of the first and second tuners, and a second joint local oscillator is shared in the IF stage of the first and second tuners. By sharing one or more joint local oscillators in separate tuners, the first and second channels of the multiple channel diversity receivers will therefore be coherent in frequency and phase and thus have common phase noise characteristics.
AUTOMATIC GAIN CONTROL (AGC)
In accordance with the present invention, the first and second RF signals are processed jointly in the multiple channel diversity receiver with respect to AGC. In particular, the respective tuners in the first and second channels of the multiple channel diversity receiver share at least one joint AGC loop. The maximum difference between the AGC feedback signal in the control loop for the first channel and the AGC feedback control signal in the control loop for the second channel is limited to a selectable maximum differential.
For example, suppose that the first RF signal in the first channel is a much stronger signal as compared to the second RF signal in the second channel. The first AGC loop amplifies the first RF signal very little, if at all, because it is already a strong signal. The second RF signal in the second channel is likely to be noisy because it is a relatively weak signal. In the prior art, the weaker noisy signal in the second channel would be greatly amplified by the AGC control loop of the second channel. Noise amplified in the AGC control loop second channel increases the noise problems and worsens the overall bit error rate when the first and second channels of the diversity receiver are combined equally.
In accordance with the present invention, the AGC control loop with the stronger first RF signal limits the maximum amount that the weaker signal is amplified in the AGC control loop with the weaker second RF signal. By limiting the AGC feedback signal in the control loop of the second channel to a maximum differential with respect to the AGC feedback signal in the control loop of the first channel, the weaker signal is not overly amplified. In such manner, the noise in the weaker noisy signal in the second channel would not be as greatly amplified by the joint AGC control loop of the second channel, as it would be in the case of an independent AGC loop.
JOINT TIMING LOOPxe2x80x94BAUD CLOCK RECOVERY
The baud clock is roughly equivalent to the data symbol clock. As in the above example for 8 VSB each data symbol is 3 bits. The baud clock timing determines the point in time when the received signal is xe2x80x9csampledxe2x80x9d to determine which of the 8 levels is represented by the current data symbol. In a diversity receiver, there is a separate baud clock recovery mechanism for each channel, each respective recovered baud clock representing the regular points in time at which the received signal is sampled to recover a data symbol in that respective channel. Known prior art techniques for recovering the baud clock include adaptive algorithms for estimating the energy at the edge of the data spectrum. In the general case, the baud clock timing will fall somewhere in between actual signal samples. An interpolator is used to interpolate between actual signal samples to obtain a signal xe2x80x9csamplexe2x80x9d at the baud clock timing.
The first and second RF signals are processed jointly in the multiple channel diversity receiver with respect to individual baud clock recovery. Although separate baud clocks are recovered for each channel, the baud clocks are recovered in a joint timing loop shared by both channels.
In accordance with the present invention, the respective front ends in the first and second channels of the multiple channel diversity receiver share a joint timing loop filter for baud clock recovery. The baud clock for each channel is synthesized in a respective phase locked loop (PLL) for each channel. Since both channels are assumed to be receiving the same signal, the received signal frequency in both channels is the same. The primary timing difference between the signals in the first and second channels is in the phase of each respective baud clock.
In accordance with the present invention, the timing loop filters of each channel are cross coupled to create a joint timing loop between both channels. By cross coupling the two PLL""s in a joint loop filter, one channel (with the stronger signal) provides a dominant influence on the frequency of the synthesized baud clock in the other channel (with the weaker signal). By sharing a joint loop filter, the baud clock PLL in both channels will tend to be frequency locked to the frequency of the stronger signal, leaving the respective PLL""s to make an individual phase adjustment for each channel. In such manner, the stronger signal in the first channel is used to determine the frequency of the baud clock for the weaker signal in the second channel.
In addition, the respective front ends in the first and second channels of the multiple channel diversity receiver share a skew corrector for baud clock recovery. A skew correction is needed when the multipath delay between the first and second RF signals in the two channels is greater than one whole baud clock period. That is, even though the frequency and phase of the respective baud clock for received signals in the first and second channels is determined in the joint timing loop, there may be whole baud skews between the two received signals. The purpose of the whole baud skew corrector is to align the received data bits in the first channel with the received data bits in the second channel.
In accordance with the present invention, a whole baud skew corrector is provided, which couples the first and second channels of the diversity receiver in a joint timing loop for baud clock recovery. In particular, after the joint loop filter of the joint timing loop settles down near steady state frequency and phase for each respective baud clock, the whole baud skew corrector is enabled.
The whole baud skew corrector computes the correlation between the first and second received signals. First and second signals without any skew (i.e., properly aligned signals) show high correlation values. First and second signals with substantial skew between the two signals show low correlation values. The first and second signals are then shifted by one whole baud period with respect to each other in a variable delay memory and the correlation is recomputed. The process of shifting the first and second received signals and computing the correlation function is repeated for various whole baud shifts in accordance with a search strategy to find the best (highest) correlation. The whole baud skew corrector shifts one or both channels in respective variable delay memories to properly align the received first and second signals in accordance with the whole baud skew delay that produced the best correlation between the first and second received signals.
JOINT PILOT LOOPxe2x80x94RF CARRIER RECOVERY
In order to demodulate (de-rotate) the received signal, the original RF carrier is recovered at the receiver. A separate RF carrier is recovered for each of the first and second channels in the diversity receiver and used to de-rotate each of the first and second received signals.
In accordance with the present invention, the respective front ends in the first and second channels of the multiple channel diversity receiver share a joint pilot loop filter for RF carrier recovery. The RF carrier signal for each channel is synthesized in a respective phase locked loop (PLL) for each channel. However, since both channels are assumed to be receiving different (multipath) versions of the same signal, the frequency of the RF carrier is the same in both channels. The difference between synthesis of the RF carrier in the two channels is the phase of each respective synthesized RF carrier in each respective channel.
In accordance with the present invention, the respective front ends of the multiple channel diversity receiver share a joint pilot loop filter in the respective PLL for RF carrier clock recovery in the first and second channels. In particular, the pilot loop filters of each channel are cross coupled to create a joint pilot loop between both channels.
By cross coupling the two RF carrier recovery PLL""s in a joint loop filter, the channel with the stronger signal provides a dominant influence on the frequency of the synthesized recovered RF carrier in the channel with the weaker signal. By sharing a joint loop filter, the phase locked pilot loops in both channels will tend to be frequency locked to the stronger signal, leaving the respective phase locked pilot loops to make an individual phase adjustment for each channel. In such manner, the stronger signal in one channel is used to determine the frequency of the recovered RF carrier signal for the weaker signal in the other channel.
FORWARD EQUALIZATION
In accordance with another aspect of the present invention, the first and second RF signals are processed jointly in the multiple channel diversity receiver with respect to forward equalization. In particular, the respective first and second channels of the multiple channel diversity receiver share a common equalization filter tap allocation scheme. That is, the number of available equalization filter taps is allocated to either the first channel or the second channel on the basis of relative need.
By way of background review, it is known to use an equalizer to mitigate the signal corruption introduced by the communications channel. An equalizer is a filter that has the inverse characteristics of the communication channel. In situations where the communication channel is not characterized in advance, or changes with time, an adaptive equalizer is used. The variable parameters (filter coefficients) of the adaptive equalizer are calculated at the receiver. After the filter parameters are properly adjusted, the equalizer filter compensates for transmission channel distortion and noise. The problem to be solved in an adaptive equalizer is how to adjust the equalizer filter parameters in order to restore signal quality to a performance level that is acceptable by subsequent error correction decoding.
A critical factor in an adaptive equalization system is to complete all the required multiplication operations within the time available: i.e., a single symbol interval. In particular, the calculation of filter parameters requires successive multiply operations for each equalizer parameter. Since a typical equalizer filter may have up to 512 filter coefficients (the number of equalizer filter parameters), the total time required to complete all the required multiplication operations with full precision often exceeds one symbol interval.
Using an equalization filter with fewer taps (coefficients) requires less computation time, but a filter with fewer coefficients is a poorer approximation to the inverse of the communication channel distortion. On the other hand, the communication channel typically introduces distortion which is clustered around certain time delays, so that most of the filter coefficients will be set to zero or near zero anyway. Therefore, an equalization filter with fewer taps could be used, provided the nonzero taps are at the correct positions.
The present invention is embodied in a tap allocation mechanism in a diversity receiver to allocate more of the available filter taps to the channel with the greater distortion and noise. The total numbers of taps to be allocated between both the first and second channels is fixed. At the start of adaptation, each equalization filter is initialized with an equal number of taps. The taps are thereafter dynamically allocated to the equalization filters of either the first or second channels on the basis of actual received signals. Thus, if both channels have equal signal path distortions, the number taps will be equally allocated to the equalization filter in each channel. On the other hand, if the signal in the first channel has greater signal path distortion than the signal in the second channel, then more equalizer taps will be allocated to the equalization filter of the first channel (and less equalizer taps allocated to the equalization filter of the second channel). In such manner, equalizer taps are efficiently allocated to the equalization filter in the channel where it is most needed.